calculating mean based on multiple variables

[Koen Appeltans]

Hello, I have a variable year containing years 1996-2014 and a variable firm_size which can take values "Small", "Medium", "large". When I generated the average of another numerical variable markup i used this code: egen average_markup_clean = mean(markup_clean), by(year). But I know want to do this per year based on the firm_size, however by() can only contain one argument. How should I encounter this problem? Thank You in advance!

[Nick Cox]

The premise is not correct. There is no bar to two arguments.

. sysuse auto, clear
(1978 automobile data)

. egen mean = mean(mpg), by(foreign rep78)

. tabdisp rep78 foreign, c(mean)

------------------------------
Repair    |
record    |     Car origin    
1978      | Domestic   Foreign
----------+-------------------
        1 |       21          
        2 |   19.125          
        3 |       19  23.33333
        4 | 18.44444  24.88889
        5 |       32  26.33333
        . |    23.25        14
------------------------------

.